Comment 1 for bug 150817

Thanks for the bug report. This particular bug has already been reported, but feel free to report any other bugs you find.

What you describe is what nautilus is supposed to do actually

"2005-05-08 Martin Wehner <email address hidden>

 * libnautilus-private/nautilus-thumbnails.c:
 (thumbnail_thread_start):
 Don't try to thumbnail files which have been modified in the
 last few seconds to avoid constantly re-thumbnailing them.
 Current cool-off period is three seconds. Fixes bug #107418."

Maybe there is something buggy in the code or your download sometime stop for over 3 seconds?